Digital microphone with over-voltage protection

ABSTRACT

The disclosure relates generally to microphone and vibration sensor assemblies (100) having a transducer (102), like a microelectromechanical systems (MEMS) device, and an electrical circuit (103) disposed in a housing (110) configured for integration with a host device. The electrical circuit includes an output driver circuit, a low drop out (LDO) regulator circuit, and an over-voltage protection circuit with improved capacity and response time.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims and benefit of, and priority to, IndianProvisional Patent Application No. 202111013641 filed Mar. 27, 2021, theentirety of which is incorporated by reference herein.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to microphones and other sensorassemblies and more particularly to digital sensor assemblies having anover-voltage protected output driver, electrical circuits therefor, andmethods of operating sensor assemblies.

BACKGROUND

Microphone assemblies comprising a transducer and an integrated circuitdisposed in a housing having an electrical interface for integrationwith a host device are known generally. Such microphones are used incell phones, personal computers, smart speakers, and internet of things(IoT) devices, among other host devices. However, in some applicationsmicrophones and other sensor assemblies can be subject to over-voltagetransients caused by energy reflections on electrical conduits likecircuit board traces that behave as transmission lines.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present disclosure willbecome more fully apparent from the following description and appendedclaims, considered in conjunction with the accompanying drawings. Thedrawings depict only representative embodiments and are therefore notconsidered to limit the scope of the disclosure.

FIG. 1 is a cross-sectional view of a sensor assembly.

FIG. 2 is a schematic block diagram of a sensor system.

FIG. 3 is a schematic diagram of a voltage regulator circuit and anover-voltage protection circuit.

FIG. 4 is a schematic diagram of an over-voltage protection circuit.

FIG. 5 is a signal diagram of the sensor assembly having an over-voltageprotection circuit.

FIG. 6 is a flow diagram representative of a method of operating asensor assembly having a high-speed and high-capacity current sinkcircuit.

Those of ordinary skill in the art will appreciate that the figures areillustrated for simplicity and clarity and therefore may not be drawn toscale and may not include well known features, that the order ofoccurrence of actions or steps may be different than the order describedor be performed concurrently unless specified otherwise, and that theterms and expressions used herein have the meaning understood by thoseof ordinary skill in the art except where different meanings areattributed to them herein.

DETAILED DESCRIPTION

The present disclosure relates generally to digital microphones andother sensor assemblies comprising a transducer and a current-sinkcircuit. The current-sink circuit is configured to prevent over-voltagetransients on an output channel of the sensor assembly.

The sensor assembly generally comprises a transducer and an electricalcircuit disposed in a housing configured to interface with a hostdevice. FIG. 1 is a cross-sectional view of a sensor assembly 100comprising a transducer 102 coupled to an electrical circuit 103disposed within a housing 110. The housing includes a base 116 and acover 118 fastened to an upper surface 120 of the base. In someimplementations, the housing shields the transducer and the electricalcircuit located within an interior of the housing from electromagneticinterference like RF noise. For this purpose, the cover can be metal orinclude a conductive portion electrically coupled to a conductiveportion of the base. The housing also includes an electrical interfacewith contacts (e.g., supply, ground, data, clock, select, etc.)configured to interface with a host device. In FIG. 1, thehost-interface is a surface-mount interface 113 located on an outersurface of the base 116 and is suitable for reflow soldering processes.In other embodiments, the host-interface can have some other formfactor, like through-hole pins, or be located on some part of thehousing.

In some sensor assemblies, like microphones, the housing includes anaperture (also called a “port” herein) connecting the interior of thehousing to the external environment. In FIG. 1, the housing port 128 islocated on the base 116 in alignment with the transducer 102. In othersensor assemblies, the port can be on some other part of the housing,like the cover or sidewall. Other sensor assemblies, like acousticvibration sensors and accelerometers among others, do not require aport.

In one embodiment, the sensor assembly is a microphone assembly and thetransducer is configured to detect acoustic signals propagated throughthe atmosphere and detected by a transducer within the housing. In otherembodiments, the sensor assembly is configured to generate an electricalsignal representative of vibrations. For example, the sensor assemblycan be configured to detect acoustic vibrations propagated through aperson's body or an inanimate object. Other sensor assemblies can beconfigured to detect vibration, pressure, acceleration, humidity, ortemperature, among other conditions. In FIG. 6, at 601, the sensorassembly generates an electrical signal representing sound or some othercondition sensed by the sensor, examples of which are described herein.The transducer may be a capacitive, piezoelectric, optical or othertransduction device implemented as a microelectromechanical systems(MEMS) device or as some other known or future device.

The electrical circuit generally comprises a processing circuitconfigured to process the electrical signal produced by the transducerand to provide the processed output signal at the host interface of thesensor assembly. In FIG. 1, the electrical circuit 103 is coupled to thetransducer 102 via leads 130 a and 130 b and to contacts on the hostinterface for this purpose. In FIG. 2, the processing circuit 275includes a signal conditioner 201 and an analog-to-digital (ADC) circuit202 configured to receive an analog signal generated by the transducerand output a digital signal representative of the analog signal. In oneimplementation, the ADC is a multi-bit delta-sigma ADC. The signalconditioner circuit can comprise a low noise amplifier, a buffer, afilter or some combination of these and other signal conditioningcircuits. The processing circuit can also comprise, optionally, adigital signal formatting circuit between the digital output of the ADCand an output terminal of the sensor assembly. The formatting circuitcan be configured to format the digital signal for a particular digitalprotocol like PDM or Soundwire, among others. Alternatively, theprocessing circuit can output a PCM format signal for output hostinterface. The electrical circuit can also include other circuitelements, depending on the transducer type and the particular sensorconfiguration, some of which are described herein.

The electrical circuit also comprises an output driver circuit (e.g., anInput/Output driver circuit) coupled to a digital output of theprocessing circuit and to the output terminal of the sensor assembly. InFIG. 2, the output driver 203 is coupled to the processing circuit andconfigured to receive a digital signal from the processing circuit andoutput the digital signal to an external device (e.g., host device), forexample, via the host-interface. Thus output 290 of the electricalcircuit is electrically coupled to a contact (e.g., the “data” contact)of the host-interface. The output driver circuit 203 is also coupled toa low dropout (LDO) regulator circuit 204 that is configured to providea regulated bias voltage to the output driver circuit 203. The output290 is coupled to the host device 299 via the host-interface of thesensor assembly and communicates with host 299 via a transmission line,lead, or trace 250. In FIG. 6, at 602, the electrical circuit outputs adigital signal at the host interface of the sensor assembly.

Reflected energy received at the output driver circuit 203 can produceundesirable over-voltage transients that can cause an error orpotentially damage the electrical circuit. Accordingly, the electricalcircuit 103 also includes an over-voltage protection circuit 206 coupledto the output driver circuit 203 and configured to sink currentassociated with the reflected energy. The over-voltage protectioncircuit 206 is configured to enable a current-sink circuit at the outputof the LDO regulator circuit 204 to sink current when a voltage at theoutput of the LDO exceeds a reference voltage. The threshold can be areference voltage related to an over-voltage specification of the outputdriver circuit. Upon detecting the over-voltage condition, theover-voltage protection circuit configures the current sink circuit tosink the reflected energy. In FIG. 6, at 603, the over-voltageprotection circuit detects an over-voltage condition at the output ofthe LDO regulator circuit. For example, the over-voltage can be causedby energy reflected back to the output driver circuit. At 604, thecurrent sink circuit is configured to sink reflected energy when anover-voltage is detected.

FIG. 3 depicts an example of an over-voltage protection circuit 206 withthe LDO regulator circuit 204. The LDO regulator circuit 204 includes areference buffer 340 coupled to the over-voltage protection circuit 206by a low pass filter 341. The reference buffer 340 is configured tooutput an over-voltage reference that is greater than a referencevoltage of the LDO regulator circuit 204, wherein the threshold is basedon the over-voltage reference. The low pass filter 341 de-couples thebuffer loop from parasitics of the over-voltage protection circuit 206and provides sufficient capacitance to keep a comparator controlledswitch 360 from being affected by transients in the current-sinkcircuit.

In FIG. 3, the schematic diagram 300 includes a voltage generator 310such as a band-gap reference or other voltage generator which feeds anindependent reference voltage to a reference buffer 340 of the LDOregulator circuit 204. The voltage generator 310 also provides thereference voltage to a reference current generator 302. The referencecurrent generator 302 may be a 1/R reference current generator that isconfigured to output various signals to bias the over-voltage protectioncircuit 206.

In FIG. 3, the over-voltage protection circuit 206 includes a currentcomparator circuit coupled to the current-sink circuit 301 and a controlcircuit 330 coupled to the current comparator and configured to change astate of the comparator when a voltage of the reflected energy exceeds athreshold. In an embodiment, the current comparator circuit enables orconfigures the current-sink circuit 301 to sink current associated withan over-voltage upon changing state. In FIG. 4, the current comparatorcircuit comprises a first current source 311, a second current source312 isolated by a cascode circuit 313, and a third current source 314.The cascode circuit 313 stabilizes a current comparator circuit outputsignal applied to the current-sink circuit 301. In FIG. 3, the currentsources are implemented using transistors.

In FIG. 2, the current-sink circuit (e.g., circuit 301 in FIG. 3) of theover-voltage protection circuit 206 is configured to sink currentassociated with an over-voltage received by the output driver circuit203 based on the output signal received from the comparator circuit. Forexample, upon activation or change of the output signal voltage from thecurrent comparator circuit, the current-sink circuit sinks or drains thecurrent from the output of the LDO regulator circuit 204 to ground.

In FIG. 3, a control circuit 330 of the over-voltage protection circuit206 is configured to provide additional current to the currentcomparator circuit when the over-voltage exceeds a threshold. Theadditional current associated with the over-voltage increases a rate atwhich the current comparator circuit changes state. In animplementation, the control circuit 330 includes a comparator-controlledswitch 360 coupled to the output driver circuit and configured toincrease a voltage applied to the current comparator circuit when thevoltage of the reflected energy exceeds the threshold, and in response,the current comparator circuit changes states in response to theincreased voltage. A control terminal of the comparator-controlledswitch 360 is coupled to the reference buffer 340 via the low passfilter 341. For example, an increased voltage at an output of LDOregulator circuit 204 causes the comparator-controlled switch 360 toincrease a resistance and thereby increase the voltage at a firstterminal of the comparator-controlled switch 360 that is connected to afirst terminal of the cascode circuit 313, which causes the outputsignal of the cascode circuit 313 to increase in magnitude and sink thecurrent at the output of the LDO regulator circuit 204. However, theresponse time of the comparator-controlled switch 360 may not besufficient to prevent over-voltage conditions caused by reflected energytransients that may appear on the output of the LDO regulator circuit204.

In some embodiments, the control circuit includes additional circuitrythat increases the response time of the over-voltage protection circuit.For example, in FIG. 3 the control circuit 330 includes a variableresistance circuit 370 coupled to the output driver circuit and to thecurrent comparator circuit. The variable resistance circuit 370 isconfigured to provide the reflected energy to the current comparatorcircuit in a relatively low resistance state when the voltage of thereflected energy exceeds the threshold. The variable resistance circuit370 is in a relatively high resistance state when the voltage of thereflected energy does not exceed the threshold. In FIG. 4, the variableresistor circuit 370 includes a first transistor 410 having a controlgate coupled to ground, a first terminal coupled to a supply voltage(VDDA shown in FIG. 3), and a second terminal coupled to a control gateof a second transistor 405 and to an output of the LDO regulator circuitvia a first capacitor 401. The second transistor 405 includes a firstterminal coupled to a first terminal 413 of the cascode circuit 313 viaa second capacitor 403 and a second terminal coupled to the output ofthe LDO regulator circuit. During a voltage spike on the output of LDOregulator circuit, the voltage at the control gate of the secondtransistor 405 will spike or rise thereby dropping the resistance of thesecond transistor which causes an output signal of the cascode circuitto approach the supply voltage VDDA thereby improving the response timeof the current-sink circuit 301.

In FIG. 5, a graph 500 of various signals from the over-voltageprotection circuit 206 is depicted. A first signal 501 depicts anexample of a voltage transient on the output of the LDO regulatorcircuit 204. A second signal 502 depicts an example output signal to thecurrent-sink circuit 301 without the variable resistor circuit 370. Athird signal 503 depicts a second example output signal to thecurrent-sink circuit 301 with the variable resistor circuit 370. Afourth signal 503 depicts a voltage signal the first terminal 413 of thecomparator circuit. The pulse in input current depicted by the firstsignal 501 at a first time is reacted to by a pulse on the fourth signal503 from the first capacitor 401. The pulse on the fourth signal 503(e.g., at the gate of the second transistor) causes the resistance ofthe second transistor 405 to drop to a magnitude that allows the pulseof the first signal 501 to couple to the first terminal 413 of thecascode circuit 313 via the second capacitor 403. This createsadditional current in the first terminal 413 that speeds up thestate-change of the current-sink circuit 301. As can be seen, the secondsignal 502 of the circuit not having the control circuit has a worseresponse time in reacting to the impulse caused by the reflected energyat the output of the LDO regulator 204 than the response time of thethird signal 503 having the control circuit 330.

In FIG. 3, the control circuit 330 includes a bandwidth control circuit361 configured to prevent the current comparator circuit fromoscillating between states when the reflected energy received by theoutput driver circuit oscillates about the threshold at a frequencygreater than a pre-set bandwidth of the bandwidth control circuit. In animplementation shown in FIG. 4, the bandwidth control circuit 361includes a third capacitor 402 coupled between the second terminal ofthe second transistor 405 and the first terminal 413 of the cascodecircuit 313. The third capacitor 402 fixes the loop-bandwidth to thepre-set bandwidth such that parasitic on other nodes are blocked fromaffecting the comparator circuit. The value of the second capacitor 403may be selected relative to the value of the third capacitor 402 suchthat the value of the second capacitor 403 is greater than the value ofthe third capacitor 402 by a factor of 8 or more. In this way, thetransient coupling by the third capacitor 402 between the output of theLDO regulator circuit and the comparator circuit is much less comparedto the coupling caused by the second capacitor 403. In some embodiments,the pre-set bandwidth is a characteristic frequency of the over-voltageprotection circuit. In some implementations, the control signal isconfigured to increase a voltage applied to the current comparatorcircuit when the voltage of the reflected energy exceeds the thresholdvoltage.

While the disclosure and what is presently considered to be the bestmode thereof has been described in a manner establishing possession andenabling those of ordinary skill in the art to make and use the same, itwill be understood and appreciated that there are many equivalents tothe select embodiments described herein and that myriad modificationsand variations may be made thereto without departing from the scope andspirit of the invention, which is to be limited not by the embodimentsdescribed but by the appended claims and their equivalents.

What is claimed is:
 1. A sensor assembly comprising: a transducerconfigured to generate an analog signal responsive to sensing changes inan environment; and an integrated circuit coupled to an output of thetransducer, the integrated circuit comprising: a signal processingcircuit connectable to a transducer of the digital sensor assembly; anoutput driver circuit coupled to the signal processing circuit; a lowdrop out (LDO) regulator circuit coupled to the output driver circuit;and an over-voltage protection circuit comprising: a current-sinkcircuit coupled to the output driver circuit and configured to sinkcurrent associated with reflected energy received by the output drivercircuit; a current comparator circuit coupled to the current-sinkcircuit; and a control circuit coupled to the current comparator andconfigured to change a state of the current comparator when a voltage ofthe reflected energy exceeds a threshold, wherein the current comparatorenables the current-sink circuit to sink current associated with thereflected energy upon changing state.
 2. The sensor assembly of claim 1,the control circuit configured to provide the reflected energy to thecurrent comparator circuit as additional current when the voltage of thereflected energy exceeds the threshold, wherein the additional currentdue to the reflected energy increases a rate at which the currentcomparator circuit changes state.
 3. The sensor assembly of claim 2, thecontrol circuit including a bandwidth control circuit configured toprevent the current comparator circuit from oscillating between stateswhen the reflected energy received by the output driver circuitoscillates about the threshold at frequency greater than a bandwidth ofthe bandwidth control circuit.
 4. The sensor assembly of claim 3, thecontrol circuit configured to increase a voltage applied to the currentcomparator circuit when the voltage of the reflected energy exceeds thethreshold.
 5. The sensor assembly of claim 2, the control circuitcomprising a variable resistance circuit coupled to the output drivercircuit and to the current comparator circuit, the variable resistancecircuit configured to provide the reflected energy to the currentcomparator circuit in a relatively low resistance state when the voltageof the reflected energy exceeds the threshold compared, wherein thevariable resistance circuit is in relatively high resistance state whenthe voltage of the reflected energy is does not exceed than thethreshold.
 6. The sensor assembly of claim 5, the control circuitcomprising a switch coupled to the output driver circuit and configuredto increase a voltage applied to the current comparator when the voltageof the reflected energy exceeds the threshold, wherein the currentcomparator circuit changes state in response to the increased voltage.7. The sensor assembly of claim 6, the control circuit comprising abandwidth control circuit coupled to the output driver circuit and tothe current comparator circuit, the bandwidth control circuit configuredto prevent the current comparator circuit from changing state toreflected energy transients received by the output driver circuit. 8.The sensor assembly of claim 1, wherein the over-voltage protectioncircuit further includes a current comparator circuit-controlled switchconfigured to output current from the current comparator circuit to theLDO regulator circuit, the current comparator circuit-controlled switchconfigured to be disabled when the current comparator enables thecurrent-sink circuit.
 9. The sensor assembly of claim 1, the currentcomparator circuit comprising a first current source and a secondcurrent source isolated by a cascode circuit, wherein the cascodecircuit stabilizes a current comparator circuit output signal applied tothe current-sink circuit.
 10. The sensor assembly of claim 1, the LDOregulator comprising a reference buffer coupled to the over-voltageregulator circuit by a low pass filter, the reference buffer configuredto output an over-voltage reference that is greater than a referencevoltage of the LDO regulator, wherein the threshold is based on theover-voltage reference.
 11. The sensor assembly of claim 1 is amicrophone further comprising a housing having a host-interface and asound port, wherein the transducer and the integrated circuit aredisposed in the housing, the transducer is a MEMS transduceracoustically coupled to the sound port, and the integrated circuit iselectrically coupled to contacts on the host-interface.
 12. Anintegrated circuit for interfacing with a transducer of a digital sensorassembly, the integrated circuit comprising: a signal processing circuitconnectable to a transducer of the digital sensor assembly; an outputdriver circuit coupled to the signal processing circuit; a low drop out(LDO) regulator circuit coupled to the output driver circuit; and anover-voltage protection circuit comprising: a current-sink circuitcoupled to the output driver circuit and configured to sink currentassociated with reflected energy received by the output driver circuit;a current comparator circuit coupled to the current-sink circuit; and acontrol circuit coupled to the current comparator and configured tochange a state of the current comparator when a voltage of the reflectedenergy exceeds a threshold, wherein the current comparator enables thecurrent-sink circuit to sink current associated with the reflectedenergy upon changing state.
 13. The integrated circuit of claim 12, thecontrol circuit configured to provide the reflected energy to thecurrent comparator circuit as additional current when the voltage of thereflected energy exceeds the threshold, wherein the additional currentdue to the reflected energy increases a rate at which the currentcomparator circuit changes state.
 14. The integrated circuit of claim13, the control circuit including a bandwidth control circuit configuredto prevent the current comparator circuit from oscillating betweenstates when the reflected energy received by the output driver circuitoscillates about the threshold at frequency greater than a bandwidth ofthe bandwidth control circuit.
 15. The integrated circuit of claim 14,the control circuit configured to increase a voltage applied to thecurrent comparator circuit when the voltage of the reflected energyexceeds the threshold.
 16. The integrated circuit of claim 12, whereinthe over-voltage protection circuit further includes a currentcomparator circuit-controlled switch configured to output current fromthe current comparator circuit to the LDO regulator circuit, the currentcomparator circuit-controlled switch configured to be disabled when thecurrent comparator enables the current-sink circuit.
 17. The integratedcircuit of claim 13, the control circuit comprising a variableresistance circuit coupled to the output driver circuit and to thecurrent comparator circuit, the variable resistance circuit configuredto provide the reflected energy to the current comparator circuit in arelatively low resistance state when the voltage of the reflected energyexceeds the threshold compared, wherein the variable resistance circuitis in relatively high resistance state when the voltage of the reflectedenergy is does not exceed than the threshold.
 18. The integrated circuitof claim 17, the control circuit comprising a switch coupled to theoutput driver circuit and configured to increase a voltage applied tothe current comparator when the voltage of the reflected energy exceedsthe threshold, wherein the current comparator circuit changes state inresponse to the increased voltage.
 19. The integrated circuit of claim18, the control circuit comprising a bandwidth control circuit coupledto the output driver circuit and to the current comparator circuit, thebandwidth control circuit configured to prevent the current comparatorcircuit from changing state to reflected energy transients received bythe output driver circuit.
 20. The integrated circuit of claim 12, thecurrent comparator circuit comprising a first current source and asecond current source isolated by a cascode circuit, wherein the cascodecircuit stabilizes a current comparator circuit output signal applied tothe current-sink circuit.
 21. The integrated circuit of claim 12, theLDO regulator comprising a reference buffer coupled to the over-voltageregulator circuit by a low pass filter, the reference buffer configuredto output an over-voltage reference that is greater than a referencevoltage of the LDO regulator, wherein the threshold is based on theover-voltage reference.